It has long been recognized that the lighter weight and better heat transfer properties make aluminum alloys the logical choice as a material for internal combustion engine blocks. However, most aluminum alloys lack wear resistance and it has been customary in the past to chromium plate the cylinder bores in the engine block with abrasion resistant coatings such as chromium, or alternately, to apply cast iron liners to the bores. It is difficult to uniformly plate the cylinder bores and as a result, plating is an expensive operation, and in the case of chromium plating, not environmentally friendly. The use of cast iron liners increases the overall cost of the engine block, as well as the weight of the engine.
Hypereutectic aluminum-silicon alloys, containing more than I2% by weight of silicon, possess good wear resistance achieved by the precipitated silicon crystals which constitute the primary phase. Because of the wear resistance, attempts have been made to use hypereutectic aluminum-silicon alloys as casting alloys for engine blocks to eliminate the need for plated or lined cylinder bores.
Typical wear resistant aluminum-silicon alloys are described in U.S. Pat. Nos. 4,603,665 and 4,969,428. U.S. Pat. No. 4,603,665 describes a hypereutectic aluminum-silicon casting alloy having particular use in casting engine blocks for marine engines. The alloy of that patent is composed by weight of 16% to 19% silicon, 0.4% to 0.7% magnesium, less than 0.37% copper, and the balance aluminum. The alloy has a narrow solidification range providing the alloy with excellent castability, and as the copper content is maintained at a minimum, the alloy has improved resistance to salt water corrosion.
U.S. Pat. No. 4,969,428 is directed to a hypereutectic aluminum-silicon alloy containing in excess of 20% by weight of silicon and having an improved distribution of primary silicon in the microstructure. Due to the high silicon content in the alloy, along with the uniform distribution of the primary silicon in the microstructure, improved wear resistance is achieved.
High pressure die casting operations have generally been used in the past to cast engine blocks. In a high pressure die casting operation, pressures in excess of 5000 psi are normally encountered and metal molds and cores are employed. Due to the high metallostatic pressures associated with high pressure die casting, conventional bonded sand cores are difficult to apply in general and cannot be employed where size exceeding 10 cubic inches is involved in conjunction with modest or major geometric shape complexity, because they do not have the strength to withstand the high pressures.
It has been found that when using high pressure die casting operations to cast engine blocks from hypereutectic aluminum-silicon alloys using a metal mold and metal core, a denuded zone, free of primary silicon, is formed at the as cast surface, because of the rapid heat extraction through the metal core. As the cylindrical surface bordering the metal core constitutes the cylinder bore in the cast engine block, the denuded condition adversely effects the wear resistance of the cylinder bore. Because of this, it has been the practice to remove up to 0.060 inch from the bore by expensive stock removal procedures, and even with the removal of that thickness, the volume fraction of primary silicon is often less than that predicted by the phase diagram. A cylinder bore with a low volume fraction of primary silicon can cause field failure of the engine, due to the decreased wear resistance.